Gas turbine engines include a compressor assembly, a combustor assembly, and a turbine assembly. The compressor compresses ambient air, which is then channeled into the combustor, where it is mixed with a fuel. The fuel and compressed air mixture is ignited, creating a working gas that may reach temperatures of 2500 to 2900° F. (1371 to 1593° C.). This gas then passes through the turbine assembly. The turbine assembly has a rotating shaft holding a plurality of circular arrays or “rows” of rotating blades. The turbine assembly also has a plurality of circular arrays of stationary vanes attached to a casing of the turbine. Each row of blades is preceded by a row of vanes to direct the working gas at an optimum angle against the blades. Expansion of the working gas through the turbine assembly results in a transfer of energy from the working gas to the rotating blades, causing rotation of the shaft.
Each row of blades is closely surrounded by a shroud ring that contains and channels the working gas. A pressurize cooling fluid, such as a portion of air from the compressor, is directed against the radially outer surface of the shroud ring (“radially” means relative to a turbine shaft rotational axis). Each shroud ring may be formed of a circular array of adjacent arcuate ring segments. A seal is needed between each pair of adjacent ring segments to prevent the pressurized cooling fluid from leaking inward between the segments, and to prevent the working gas from leaking outward. This is conventionally provided by a metal strip seal mounted in a narrow slot on each side of the gap between ring segments. The strip seal spans the gap, blocking leakage of gas and allows for some differential thermal expansion between adjacent shroud segments.
Each vane includes an airfoil with a radially outer platform connected to the turbine casing. The vane may also have an inner platform connected to an inner coolant plenum. The outer platforms in a given row of vanes are mounted adjacent to each other as segments in a circular array, defining an outer shroud ring. The inner platforms are likewise mounted adjacent to each other in a circular array, defining an inner shroud ring. These outer and inner shroud rings define a flow channel between them which channels the working gas over the stationary vane airfoils. A seal is needed between each pair of adjacent outer platforms, between each pair of inner platforms, as well as between each pair of adjacent shroud rings.
Gas turbines are being designed with ever-increasing working gas temperatures to provide high efficiency. This evolution is enabled by high-temperature component materials such as ceramic matrix composites (CMC) and other ceramics for components exposed to the working gas, including shroud ring segments. Differential thermal expansion between CMC ring segments and metal support structures occurs during a range of turbine operating conditions. This causes cyclic changes in the seal gap size between the ring segments.
Conventional strip seals have clearance in their slot depth to allow expansion and contraction of the gap. Due to the relative brittleness of ceramics compared to metal, such conventional sealing methods are not desirable in ceramics, because relatively narrow and deep seal slots can create stress concentrations that are not well tolerated by ceramics.